Voice notes on iOS app

I believe now the best way to do this is through Google Keep that sync with Joplin somehow. Folks were able to implement it. The point is Google Keep has nice shortcut for quick note recording.

Also check out this topic:

I personally was thinking about doing it so it does not involve Google and you can read about it here (no solution though, just thoughts).
Topic also contains some Google Keep discussions as far as I recall.